1. Field of the Invention
The present invention relates to a valve system for an engine. In particular, the present invention relates to an improved valve system for an engine, wherein a rocker arm is interlocked with an engine valve and is supported by a rocker shaft fixedly supported by a cylinder head such that it is rockable with the rotation of a camshaft of the engine.
2. Description of Background Art
A valve system of this type has been known, for example, from Japanese Patent Laid-open No. Hei 2000-110516.
The above-described background art valve system is of an overhead cam type, wherein a camshaft is disposed over a cylinder head. In this valve system, to transmit rotational power of the crankshaft to the camshaft at a reduction ratio of 1/2, it is required to ensure a space on one end of the crankshaft for disposing power transmission means, such as a cam chain. This results in the width of the engine increasing in the direction along the axial line of the crankshaft. Accordingly, the weight of the engine is increased.
In view of the foregoing, the present invention has been made, and an object of the present invention is to provide a valve system of an engine, which is capable of reducing the size and weight of the engine.
To achieve the above object, according to a first aspect of the present invention, a valve system for an engine includes a crankshaft rotatably supported by a crankcase such that it is connected to a piston slidably fitted in a cylinder bore. A camshaft has an axial line parallel to an axial line of the crankshaft and is interlocked with the crankshaft at a reduction ratio of 1/2. A rocker arm is interlocked with engine valves and is supported by a rocker shaft fixedly supported by a cylinder head such that it is rockable with the rotation of the camshaft. The camshaft disposed on one side of the crankshaft is rotatably supported by the crankcase. The posture of the rocker shaft is set to have an axial line crossing the axial line of the crankshaft on the projection plane perpendicular to an axial line of the cylinder bore. An input arm extending on the projection plane from the rocker arm to the camshaft side is provided on the side of the rocker arm which is rockably supported by the rocker shaft. A pull rod reciprocating in the axial direction with the rotation of the camshaft is connected to the input arm in order to rock the rocker arm in the valve opening direction when the pull rod is moved to the camshaft side.
With this configuration, since the posture of the rocker shaft is set to have an axial line crossing the axial line of the crankshaft on the projection plane perpendicular to an axial line of the cylinder bore, a space required for disposing the rocker shaft, the rocker arm, and the like constituting part of the valve system can be made smaller in the direction along the axial line of the crankshaft. Also, since the input arm extends from the rocker arm to the camshaft disposed on one side of the crankshaft, and the pull rod reciprocating in the axial direction with the rotation of the camshaft is connected to the input arm, it is not required to enlarge the width of the cylinder head in the direction along the axial line of the crankshaft for disposing the pull rod. This results in the possibility of reducing the size and weight of the engine in the direction along the axial line of the crankshaft.
To achieve the above object, according to the second aspect of the present invention, a valve system for an engine includes a crankshaft which is rotatably supported by a crankcase such that it is connected to a piston slidably fitted in a cylinder bore. A camshaft has an axial line parallel to an axial line of the crankshaft and is interlocked with the crankshaft at a reduction ratio of 1/2. A first rocker arm is interlocked with first engine valves and a second rocker arm is interlocked with second engine valves different in kind from the first engine valves. The first and second rocker arms are respectively supported by a first rocker shaft and a second rocker shaft, which are fixedly supported by a cylinder head, such that they are rockable with the rotation of the camshaft. The camshaft is disposed on one side of the crankshaft and is rotatably supported by the crankcase. The first rocker arm corresponding to a pair of the first engine valves and the second rocker arm corresponding to a pair of the second engine valves are disposed in the cylinder head such that they face the combustion chamber. The first and second rocker arms are rockably supported by the first rocker shaft and the second rocker shaft, respectively. The first and second rocker shafts have axial lines crossing the axial line of the crankshaft on the projection plane perpendicular to an axial line of the cylinder bore and are disposed in parallel to each other. A first input arm and a second input arm extend on the projection plane from the first rocker arm and the second rocker arm to the camshaft side and are provided on the first rocker arm and the second rocker arm, respectively. A push rod reciprocating in the axial direction with the rotation of the camshaft is connected to the first input arm in order to rock the first rocker arm in the valve opening direction when the push rod is moved on the side opposite to the camshaft. A pull rod reciprocating in the axial direction according to the rotation of the camshaft is connected to the second input arm in order to rock the second rocker arm in the valve opening direction when the pull rod is moved on the camshaft side.
With this configuration, since the pair of first engine valves and the pair of second engine valves are disposed in the cylinder head such that they face the combustion chamber, it is possible to improve the suction efficiency and increase the output torque in a low speed rotational range. Since the postures of the first and second rocker shafts are set to have axial lines thereof crossing the axial line of the crankshaft on the projection plane perpendicular to the axial line of the cylinder bore, a space required in the cylinder head for disposing the first and second rocker shafts, the first and second rocker arms, and the like constituting part of the valve system can be made smaller in the direction along the axial line of the crankshaft. Furthermore, since the input arms extend from the first and second rocker arms to the camshaft disposed on one side of the crankshaft and the pull rod and the push rod reciprocating in the axial direction according to the rotation of the camshaft are connected to the input arms of both the rocker arms, it is not required to enlarge the width of the cylinder head in the direction along the axial line of the crankshaft for disposing the pull rod and the push rod. Accordingly, a drive system between the camshaft and the rocker arms can be disposed in good balance. This results in the possibility of reducing the size and weight of the engine in the direction along the axial line of the crankshaft.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.